Up to 88 anti-government protesters were reportedly shot dead by security forces in Syria yesterday.
As the death toll rose it became clear that it was the bloodiest single day since the beginning of the uprising against the regime.
More bloodshed is expected on Saturday as large crowds are expected to attend funerals across the country.
Tens of thousands were said to have taken to the streets as demonstrators called for a ‘day of reckoning’. In total about 300 have died since the unrest began on March 18.
President Barack Obama condemned Friday's violence and accused President Assad of seeking help from Iran.
He said: 'This outrageous use of violence to quell protests must come to an end now. Instead of listening to their own people, President Assad is blaming outsiders while seeking Iranian assistance in repressing Syria's citizens.'
A Syrian human rights campaigner warned of more potential violence on Saturday. He said: 'The funerals will turn into vehement protests, like past funerals.
'When you have security services who are thugs it is difficult to think that they will not shoot at the crowds. Another cycle of funerals and demonstrations is likely to follow.'
Although Western journalists are banned from Syria and there was no way of verifying yesterday’s events, videos showing the violence were quickly posted on YouTube.
They showed demonstrators being fired on in towns across the country.
The worst death tolls were said to be in a village in the south – where protests against President Bashar Assad first began – and a suburb of the capital Damascus.
There were also protests in Cyprus, where several hundred from the large Syrian community gathered to demonstrate outside their embassy in Nicosia.
Last night, as the gravity of the situation became clear, Foreign Secretary William Hague said he was ‘extremely concerned’ by reports of deaths and casualties. He also urged restraint.
Bullets and tear gas were fired at pro-democracy demonstrations across the country, with a young boy among the dead, as President Assad’s authoritarian regime continued its brutal attempts to snuff out defiance.
Protesters flooded into the streets after Muslim prayers in at least five cities, including the capital Damascus – a sign that deadly crackdowns have failed.
‘Bullets started flying over our heads like heavy rain,’ said one witness in Izraa, a village in the southern province of Daraa, the region where the uprising began some six weeks ago.
The protest movement has been the gravest challenge against the autocratic regime led by Assad, who inherited power from his father 11 years ago in one of the most rigidly controlled countries in the Middle East.
Tens of thousands of Syrians were protesting in the Damascus suburb of Douma, the central city of Homs, Banias, on the coast, the north-eastern Kurdish region and Daraa.
Witnesses said they saw at least five corpses at the Hamdan hospital outside the capital. All had suffered gunshot wounds.
In Daraa, other witnesses said at least 10 people were killed when protesters marched in front of the mayor’s office. They said an 11-year-old was killed. ‘Among the dead was Anwar Moussa, who was shot in the head. He was 11,’ said a witness, who would not be named.
The protest movement has crossed a significant threshold in recent days, with increasing numbers now seeking the downfall of the regime, not just reforms.
The crackdown has only emboldened protesters, who are enraged over the many deaths.
Activists had promised that Friday’s protests would be the biggest yet against the regime.
The president has been trying to defuse the demonstrations by launching a bloody crackdown along with a series of concessions, most recently lifting emergency laws that gave authorities almost boundless powers of surveillance and arrest.
Amnesty International said Syrian authorities 'have again responded to peaceful calls for change with bullets and batons.'
Malcolm Smart, Amnesty International's Middle East and North Africa director, said: 'They must immediately halt their attacks on peaceful protesters and instead allow Syrians to gather freely as international law demands.'